The present invention relates to communications systems, and more particularly to a repeater for use in a wireless communications system. The invention concerns the provision of polarization diversity and pre-detection equal gain combining to reduce the deleterious effects of multipath on the receive performance of a repeater in a wireless communications system.
Diversity has been used and/or proposed for use in radio communications systems in a number of forms. Generally speaking, the term diversity refers to a method of signal extraction by which an optimum resultant signal is derived from a combination of, or selection from, a plurality of transmission paths, channels, techniques or physical arrangements. A number of types of diversity are generally known in radio frequency communications systems, including space diversity, polarization diversity, frequency diversity and time diversity. Each of these represents an arrangement by which a choice can be made between signals.
Prior U.S. Pat. Nos. 5,280,472 and 5,513,176 respectively describe a CDMA micro-cellular telephone system and distributed antenna system therefor, and a dual distributed antenna system, which use a time-delay type of diversity, in which delay is used in one path to improve multi-path performance in distributed antenna systems. These patents also generally describe a number of other CDMA (code division multiple access) techniques for cellular mobile systems, as well as briefly describing some of the other types of diversity mentioned above.
However, heretofore, polarization diversity and pre-detection, equal gain combining have not been proposed for improving the receive performance of a repeater in a wireless communications system.
In radio receivers, diversity combining, that is, combining of the two or more signals selected for the diversity reception, can be implemented either before or after detection of the received communications signal. However, in the case of a repeater, no detection is used, since a repeater merely xe2x80x9cboostsxe2x80x9d the signal from the remote users and retransmits it to a base station. Therefore, no modulation or demodulation takes place in a repeater. Thus, the present invention recognizes that pre-detection techniques can advantageously be employed in a repeater.
The usual approach to receiving two diversity path signals is to use two separate antenna elements that are spatially separated. These two separated antennas provide two versions of the desired receive signals with statistically independent multipath characteristics. The statistical independence of the multipath in each received signal is the basis for multipath improvement in a diversity combining system.
However, equal gain combining of the diversity signals is complicated by the problem of aligning the phase of the two diversity paths, since the relative phase of the desired signals is dependent on the location of the signal source with respect to the two diversity receive antennas. This problem can be further complicated when the signal source is a mobile source, since the phase variation is no longer constant or stationary.
Accordingly, is a general object of the invention to use polarization diversity and equal gain combining to improve the receive performance of a repeater in a wireless communication system, while overcoming the above-noted problems.
We have recognized that the above-stated problem of equal gain combining with a mobile signal source may be overcome by the use of polarization diversity in a repeater. It is well documented that the vertical and horizontal field components in a communications link are highly uncorrellated. We have found that by using receive antennas that have the same phase center and orthogonal polarizations, the problem of location-induced phase variation is eliminated. Therefore, we can use an equal gain combiner which utilizes a fixed phase adjustment that is dependent only on the fixed phase differences of the repeater equipment, and not upon the changing location of the mobile signal source.
Briefly, and in accordance with the foregoing, a repeater diversity system comprises a main null antenna having a given phase center and polarization for receiving a communications signal from a remote signal source; a donor antenna for transmitting a signal to a base station; a diversity null antenna having the same phase center as the main null antenna and a polarization orthogonal to the polarization of the main null antenna; a combining network coupled to the main null antenna and the diversity null antenna for combining the signals therefrom; and an uplink channel module coupled with said combining network for delivering diversity combined receive signals to said donor antenna.